1. Field of the Invention
The present invention relates to the synthesis of methylaluminoxanes which are useful as cocatalysts in the homogeneous polymerization and copolymerization of olefins and/or dienes in conjunction with metallocene compounds, e.g., such Group IVB compounds as titanium, zirconium, and hafnium.
2. Description of the Prior Art
Methylaluminoxane, prepared by carefully controlled partial hydrolysis of trimethylaluminum, is useful as a cocatalyst in conjunction with certain Group IVB compounds, such as dicyclopentadienylzirconium dichloride and racemic ethylenebis(indenyl) zirconium dichloride, in the homogeneous polymerization of olefins. Such catalyst systems, discovered by W. Kaminsky, are highly efficient and have been the subject of much interest in recent patent and journal literature, including the following: U.S. Pat. Nos. 4,404,344, 4,452,199, 4,544,762, and 4,665,208; J. Poly. Sci.: Poly. Chem. Ed., Vol. 23, page 2117(1978); Angew. Chem. Intl. Ed. Engl., Vol. 15, page 630(1976) and Vol. 19, page 390(1980); Makromol. Chem., Rapid Commun., Vol. 4, page 417(1973) and Vol. 5, page 225(1984); and J. of the Amer. Chem. Soc., Vol. 106, page 6355(1984). The known schematic schemes for the synthesis of methylaluminoxane typically show one or more of the following disadvantages: long reaction times (with hydrated salts); low yields of the methylaluminoxane (50% or lower); the potential for explosions resulting from runaway reactions; low temperatures (-10.degree. C. and below) in order to obtain optimum yields; poor batch-to-batch reproducibility; the use of exotic and expensive raw materials (for example, dimethylgalliumhydroxide, as described in the Sinn et al. publication mentioned below); or the use of unusual or complicated reactors (for example, an autoclave reactor that incorporates a milling action, as described in the Sinn et al. article cited below, an ultrasonic reactor, as described in U.S. Pat. No. 4,730,071, or the use of a high speed, high shear-inducing impeller, as described in U.S. Pat. No. 4,730,072).
At a symposium given in Hamburg, West Germany, in September, 1987, H. Sinn et al. described several new methods of preparing methylaluminoxane. Results of the symposium are given in the following citation: H. W. Sinn et al., Transition Metals and Organometallics as Catalysts for Olefin Polymerization, W. O. Kaminsky et al., eds., Springer-Verlag, New York, Proceedings of an International Symposium, Hamburg, FRG, September 21-24, 1987, pages 257-268. On page 259 of this publication several unexpected observations are detailed in regard to the use of ice in the synthesis of methylaluminoxanes. However, page 262 does contain a very cursory mention that tetraisobutyldialuminoxane reacts with "a repeatedly added excess" of trimethylaluminum by giving off triisobutylaluminum that is distilled out of the reactive vessel together with the excess of trimethylaluminum. The residue is said to be "an oligomeric aluminoxane". No data is provided in regard to this ill defined product nor is its performance as a cocatalyst in olefin polymerization described in the reference.